1. Field of the Invention
The present invention is directed to a heart stimulator having two unipolar electrodes respectively arranged in the atrium and ventricle of a heart, for detecting atrial and ventricular activity and for pacing the heart at those locations.
2. Description of the Prior Art and Related Application
Pacemakers which supply pacing pulses both to the atrium and to the ventricle of a heart, as well as sensing cardiac activity in each of those chambers, and which supply pacing pulses only in the absence of a natural heartbeat are widely used for treating patients with bradycardia. Such pacemakers are known as DDD pacemakers, and require two electrodes, respectively disposed in the atrium and ventricle, each with its own lead.
By supplying artificial stimulation pulses from the pacemaker only in the absence of a natural heartbeat, battery current is conserved. Moreover, the ability to detect an evoked response, following an artificially generated stimulation pulse, permits the energy content of the pulse to be adjusted so as to be just at the capture threshold, i.e., having the minimum energy content which is necessary to evoke a pacing response in the heart. This avoids the generation of pulses having an energy which is higher than necessary to produce the desired response, thereby further contributing battery conservation.
Such evoked response detection requires relatively complicated electronic circuitry compared to conventional pacemakers, because such circuitry must, within a few tens of milliseconds after the delivery of an artificial pacing pulse, having an amplitude on the order of one volt sense/detect, a cardiac signal on the order of some millivolts.
Two basic approaches have been undertaken in the art to sense cardiac activity within the heart and to deliver stimulation pulses, generated by circuitry contained within an implanted pacemaker enclosure, to the heart in vivo. One approach has been to use two separate unipolar electrode leads, one having an electrode disposed in the atrium and the other having an electrode disposed in the ventricle. Pacing and detection than takes place between the respective electrode and the metallic pacemaker enclosure. The other basic approach has been to use two bipolar electrode leads respectively disposed in the atrium and the ventricle. A bipolar lead carries two electrodes or electrically active surfaces, such as an electrode ring and an electrode tip. Both the electrode ring and the electrode tip are disposed in the heart.
In the case of unipolar systems, it is possible that interference may be sensed along with the cardiac signal, such interference arising on, or related to, the metallic pacemaker enclosure. Although the bipolar approach minimizes the presence of such interference in the final signal, bipolar electrodes are more complicated than unipolar electrodes, because of the necessity of having at least two electrical conductors therein.
A heart stimulator is disclosed in co-pending U.S. patent application Ser. No. 467,267, filed Jun. 6, 1995 ("Heart Stimulator," Hoegnelid et al), that application being a continuation of Ser. No. 147,744, filed Nov. 4, 1993 and now abandoned. The teachings of this co-pending application are incorporated herein by reference. This copending application discloses a heart stimulator having a pulse generator and an electrode system which contains at least one bipolar electrode with one pole arranged in the atrium and one pole in the ventricle, or at least two unipolar electrodes respectively arranged in the atrium and ventricle, for detecting atrial and ventricular activity, and having an atrial measurement unit arranged to measure a signal between the two poles of the bipolar electrode, or between the two unipolar electrodes, and a ventricular measurement unit arranged to measure a signal between the ventricular pole (or electrode) and the stimulator housing. This co-pending application is owned by the same assignee (Pacesetter AB) as the subject matter of the present application.